A number of sensors have been proposed in which the output signal is transmitted along an optical fibre. In some cases, the fibre itself is used as the transducer element, while others use an optical effect.in a material which is addressed by an optical fibre to enable that effect to be sensed. The latter group includes mechanical shutter/mirror configurations, for example U.S. Pat. No. 4,547,728, and devices using optically active liquid crystals. Grattan et al (Rev Sci. Instrum 57 (6), 1175) have described a sensor using a transducer constructed from a piece of doped glass whose absorption profile is measurably temperature-dependent over a range of interest. The device operates in a transmission configuration and also utilizes a piece of neodymium glass to generate a reference wavelength. U.S. Pat. No. 4,671,651 to Toyoda et al proposes the use of the semiconductor compound CdInGaS.sub.4 which absorbs various amounts of light of specific frequencies as a function of temperature.
A serious disadvantage of transmission mode sensors of this type is the great accuracy required in their construction to ensure that the parts are closely enough aligned that at least an adequate proportion of the light received from the incident fibre reaches the endface of the detection fibre. The problem remains if the device is converted to reflection mode in which a single fibre transmits both the incident and detected light: see, e.g., the aforementioned U.S. Pat. No. 4,671,651 and U.S. Pat. No. 4,689,483 to Weinberger. In this case, even slight misalignment of the reflector, which can easily arise during assembly or in use, can seriously diminish or eliminate the received signal.
U.S. Pat. No. 4,575,259 to Bacci et al discloses a slightly different arrangement, viz an optical fibre thermometer that utilizes the temperature dependent absorption characteristics of a thermochromic solution. Light enters the sensing probe by way of a first optical fibre after which it passes through the thermochromic solution and is reflected by a reflective substance. Some of the reflected light is captured by a second fibre which is connected to either a beam splitter or a star coupler to form two separate beams of light. The beams of light are then routed to separate filters and photodetectors for comparison and ultimate calculation of the temperature.
U.S. Pat. No. 4,673,299 to Dakin discloses a temperature sensing assembly that includes a doped optical fibre that absorbs transmitted light as a function of temperature. Unlike the previously described references, the Dakin apparatus does not use a reflective surface to return the incident light. Instead, naturally occurring back-scattered light is routed to a wavelength separator and detection elements. Respective lasers produce light of different wavelengths to compensate for any variation in dopant concentration. By comparing the detector reading with the time it is detected, it is possible to obtain the temperature at any location along the optical fibre. A second embodiment presents a similar design with the exception that one laser is used in conjunction with a light-emitting fibre and a wavelength filter. The sensing fibre is doped with a material that partially absorbs the incident light and in turn produces fluorescent light covering two wavelength bands. The wavelength filter separates these bands and the relative intensities are compared with a ratiometer.
Back-scattering, in this case from a matrix of randomly distributed chips of semiconductor crystals, is also relied upon in U.S. Pat. No. 4,288,159. U.S. Pat. No. 4,652,143 to Wickersheim et al discloses a temperature measurement apparatus in which a surface of interest is coated with a temperature sensitive luminescent material. When excited by incident light from a pulse generator and lamp, the luminescent material emits light of a particular wavelength. Some of this light is captured by the same optical fibre and this light is routed to a detector. Other types of optical temperature probe are disclosed in U.S. Pat. Nos. 4,176,551 to Hammer et al and 4,437,761 to Kroger.
U.S. Pat. No. 4,566,753 discloses an optical coupler in which a graded index rod lens (GRIN lens) is used as the medium through which the fibres are coupled. Three fibres are coupled to three other fibres. Each fibre possess a tapered portion that is centrally connected to the respective end faces of the GRIN lens. The advantage of using the GRIN lens is that a high quality coupling is achieved without the need for accurate relative alignment of the fibre optic cables.
The need to optimise collection of the signal at the return fibre-becomes all the more critical when the sensor is being employed as one of several sensors in an optical fibre network. Such networks have been proposed in which single incident and detection fibres are communicated to multiple optical components. e.g. sensors, by means of an ordered fibre tree assembled with fibre optic couplers. Since substantial attenuation occurs and inevitable losses arise as each signal traverses the tree out to each sensor and back again, including typically half-power attenuation on each branch of each coupler, it is desirable that further losses be minimised at each sensor.
A known technique for distinguishing returning pulses in networks of the kind just mentioned relies on time separation of the pulses arising from distinct lengths of fibre to each sensor. However, in and out transit times and amplitudes are also affected by ambient conditions, especially temperature: in order to enhance sensitivity and reduce the frequency of recalibration, it has been proposed that the incident light comprise alternate signals of close but discrete wavelengths, one comprising the test wavelength and the other a reference wavelength. Such is described, e.g., in U.S. Pat. No. 4,673,299 to Dakin. Differential analysis is then employed to reduce the effect of ambient conditions. However, the present inventors' experience is that ambient conditions still unduly influence results and result in an accuracy below the level desired for many practical applications of optical fibre networks.
U.S. Pat. No. 4,409,476 discloses a temperature measuring apparatus that utilizes a number of remotely placed optical temperature sensors. A drive unit operates a number of light emitting diodes (LEDs) that send light pulses to the temperature sensors. The temperature sensors contain a temperature-sensitive photoluminescent material which emits light of a particular wavelength when excited by incident light. The emitted light is routed to a receiver unit that measures the intensity of the light for the eventual calculation of temperature.